1. Field of the Invention
The present invention relates to a recording element for ejecting drops of liquid such as ink drops to a surface to be printed, and a recording device which uses such element.
2. Description of the Related Art
A typical recording method for printing by ejecting drops of liquid, more specifically ink drops onto a surface to be printed uses nozzles. The nozzle type recording method to the date includes xe2x80x9con-demandxe2x80x9d type and xe2x80x9ccontinuous flowxe2x80x9d type.
The on-demand type recording is a method that prints by ejecting ink intermittently from nozzles in response to recording information, exemplary types of this method include xe2x80x9cpiezoelectric vibratorxe2x80x9d type and xe2x80x9cthermalxe2x80x9d type. The piezoelectric vibrator type operates, by applying pulsed voltage to the piezoelectric elements disposed in vicinity of ink chamber to distort it, for varying the liquid pressure of the ink in the ink chamber to eject ink drops from the nozzles to perform recording of dots on a recording medium paper. And the thermal type on-demand recording operates, by heating ink with a heater element provided in the ink chamber to form bubbles thereby to eject ink drops from nozzles to perform recording of dots on a recording medium.
On the other hand, in the continuous flow type, some pressure is applied to the ink to eject continuously from the nozzle, while at the same time vibration is applied by a vibrator such as the piezoelectric element to cause the projected ink column to be liquid drops, and charging and deflecting are selectively performed to the ink drops for recording.
FIG. 21 shows the overview of the piezoelectric vibrator type printing: a nozzle 72 is provided in one side of an ink chamber 71, while a piezoelectric element 73 is provided outside in the opposite side, and ink 74 is fulfilled in the ink chamber 71. By applying voltage to the piezoelectric element 73, which deforms such that its volume within the ink chamber 71 is to be minimized, to thereby increase the pressure in the ink chamber 71 to eject ink from the nozzle 72. In such printing method the diameter of the ejected ink drops will be determined mainly by the diameter of the nozzle 72.
FIG. 22 shows the overview of the thermal type printing: a nozzle 82 is provided in one side of an ink chamber 81, while a heater element 83 is provided within the ink chamber 81, and ink 84 is fulfilled in the ink chamber 81. By applying voltage to the heater element 83 to heat the heater element 83, ink nearby the heater element 83 is caused to be heated to form bubbles, to thereby increase the pressure in the ink chamber 81 to eject ink from the nozzle 82. In this method, similar to the piezoelectric vibrator type printing, the diameter of the ejected ink drops will be determined mainly by the diameter of the nozzle 82.
The resolution of the image to be printed in a variety of recording type had been required to be 300 dots per inch (dpi) in the prior art; recently, higher resolution such as 600 to 720 dpi has been required. In order to satisfy such a demand, the diameter of the dot on a recording medium paper has to be smaller, this follows that the diameter of the nozzle has to be smaller such that the diameter of the ink drops ejected become smaller.
However, when the diameter of the nozzle become smaller, blocking of nozzles by particles or dust, blocking of nozzles by dried ink surface in the nozzle, and further the change of the ink ejection direction by sticked ink sludge in the circumference of the nozzle may occur easily and some defects of the printing quality on the recording medium may be resulted. Therefore the diameter of the nozzle may not be small enough to the size needed to form a dot size corresponding to the required resolution.
There have been proposed some recording method for recording by ejecting ink drops onto the surface to be printed by using vibration or acoustic wave instead of nozzle in order to overcome such problems as cited above.
The first of the recording method without using nozzles in the prior art, as disclosed in the U.S. Pat. No. 4,308,547, is the printing method in which a piezoelectric element shell with concaved spherical surface is disposed in the ink, and voltage is applied thereto via electrodes. In this method, longitudinal longitudinal wave radiated from the piezoelectric element shell to the ink is concentrated to a point in a ink free surface for ejecting drops from the ink free surface.
Moreover, a similar type of printing as disclosed in the Japanese Published Examined Patent Application No. Hei 6-45233 as a method for improving the productivity and the fine structured precision, is the one in which a spherical concaved recess is provided on a substrate such as glass to form an acoustic lens, and a vibrator comprised of a piezoelectric element and electrodes for applying voltage to the piezoelectric element is formed on the other side of the substrate, then the vibrator is disposed in the ink. In this method, as similar to the method disclosed in the above cited U.S. Patent, the vibration from the vibrator is emitted as longitudinal wave in the ink, concentrated into a point on the ink free surface by means of the concaved acoustic lens for ejecting drops from the ink free surface.
In the Japanese Published Unexamined Patent Application No. Hei 3-200199, a method is disclosed in which a phased Fresnel lens in the form of thin film plate is provided on a substrate for a focusing lens less expensive but sharper. The phased Fresnel lens operates as a lens that plane incident wave is diffracted at a plurality of thin film plate disposed annularly spaced apart at a distance, then a plurality of radiated diffracted waves are combined in a point on the ink free surface to become maximum amplitude.
As mentioned above, the first of the recording method without using nozzles converges the vibration emitted from the vibrator into one point on the ink free surface to eject ink drops. The acoustic lens is made by forming a lens by the piezoelectric element itself, or by using a phased Fresnel lens for overlaying the same phase, or by using concaved lens. It should be noted that in the acoustic lens, since the relationship between the lens form and the convergence and divergence of wave is the reverse of that of the optical lens, a concaved lens should be used; the wave will be diverged with a convex lens.
Here, the diameter of ejected drop is approximately equal to the diameter of the converged bundle when longitudinal waves propagated in the ink are converged onto the ink free surface, and the converged diameter xe2x80x9cdxe2x80x9d will be dxcx9cF/f, where xe2x80x9cfxe2x80x9d is the driving frequency of the vibrator, xe2x80x9cFxe2x80x9d is the aperture value (F value) of the lens. The relation between the wavelength of the longitudinal waves propagated in the ink xe2x80x9cxcexxe2x80x9d and its propagation speed xe2x80x9cvxe2x80x9d and the driving frequency xe2x80x9cfxe2x80x9d of the vibrator will be: v=fxc2x7xcex.
Thus, when for example, very fine ink drops of the drop diameter of approximately 15 micron is to be ejected, if the F value of the lens is 1, then the prior propagation speed of the longitudinal wave in the low viscosity water-based ink is approximately 1500 meters/sec., thus the driving frequency of the vibrator has to be set to a very high frequency such as approximately 100 MHz. Since the F value of the lens is, in practice, quite difficult to be significantly small due to many problems, in general, for the drop diameter xe2x80x9cdxe2x80x9d to be smaller, the vibrator is to be driven at a higher frequency.
As mentioned above, in the first printing method without nozzles as a plurality of vibrators have to be driven at a high frequency of near 100 MHz, a cost-related problem that the driving means becomes expensive may be resulted in, while other significant problems such as the change of the drop diameter by the variation of the ink viscosity due to the heating, or the blocking of ink ejection by the dried or solidified ink itself within a recording element, may occur.
The heating may be caused by the nature that the longitudinal wave propagated in the ink is highly attenuated in the ink if driven at higher frequency. Thus, the higher the frequency used, the larger the amount of energy absorbed in the ink due to the attenuation, and the larger the amount of heat.
In case of the recording element without using nozzles in the prior art, when this problem is to be overcome there are no way other than that a cooling mechanism is provided to the recording element, this leads to the growth of the size of the recording device and of the manufacturing cost. Therefore, a need has been existed for a recording element which may eject smaller ink drops without nozzles with lower frequency drive, and which generates less heat, as well as operates at higher energy efficiency.
As the second of the recording method without using nozzles, there is a method for ejecting ink drops by converging the vibration generated by a vibrator into a point on the ink free surface by using acoustic horn.
For example, in the above mentioned U.S. Pat. No. 4,308,547, there is disclosed that by converging vibration by an acoustic horn formed on the vibrator instead of the concaved lens, the vibration is applied to the ink thin film transported on a belt which is in contact with the tip of the acoustic horn for ejecting ink drops. Here the ejection force is generated at the tip of the acoustic horn.
Also, the Japanese Published Unexamined Patent Application No. Hei 4-168050 discloses, as shown in the FIG. 23, an acoustic collector 95 provided on a piezoelectric element 91. The acoustic collector 95 is formed on a vibrator 94 comprised of a piezoelectric element 91 and the electrodes 92 and 93 for applying voltage thereto, and is disposed in an ink reservoir 99 supporting ink 96. The acoustic collector 95 is in a form slightly tapered, as shown in the figure. In this example, the distortion incident from the bottom of the acoustic collector 95 is propagated in the acoustic collector 95 while its amplitude is amplified, the waves of a large amplitude which has been collected by the acoustic collector 95 hits the ink 96 so that the longitudinal wave thus generated will raise the ink free surface 97 to eject ink drop 98.
However, it should be noted that, in the specification of the U.S. Pat. No. 4,308,547 and the Japanese Published Unexamined Patent Application No. Hei 4-168050, either an acoustic collector or an acoustic horn is said to be used but its size, driving condition, and vibrating mode, as well as whether or not smaller drop may be obtained, are not mentioned.
Concerning the acoustic horn, in the field of acoustics in general, it is well known that for obtaining ejection force by vibrating the tip of collector (or horn) in large amplitude by resonance, the length (height) of the collector in the direction of vertical cross-section should be a multiple of the integer of the xc2xd wavelength of vibrating wave.
Thus, in the method of the prior art as shown in FIG. 23, in order to make the acoustic collector 95 smaller for manufacturing high density recording element, the wavelength of the vibrating wave has also to be shorter, thus the driving frequency of the vibrator 24 is required to be higher.
Therefore, in order to achieve a recording element of practical density, the vibrator is needed to be driven at a higher frequency ranging approximately from a few tens to 100 MHz, as similar to the first recording method without using nozzles. This follows that the attenuation of energy in the ink will be large and that the driving circuit will be expensive. Also, it is difficult in practice to securely transport ink thin film by a belt in contact with the tip of an acoustic horn, as taught in the specification of U.S. Pat. No. 4,308,547, and the diameter of ejected drop would not be stabilized.
There is the third recording method without using nozzles which uses electrostatic force as ejection force, i.e. electrostatic extraction method, and a variation which uses vibration for forming ink meniscus (ink protuberance) is known.
For example, in the Japanese Published Unexamined Patent Application No. Sho 62-222853, there is disclosed that a recording needle is projected from the ink surface to apply supersonic energy transmitted axially. According to the patent publication, because of acoustic streaming phenomenon, some ink in contact with the recording needle moves toward the direction to the tip of the needle to form ink meniscus in a convex form at the tip of the needle. In this state, electrostatic field is applied between the recording needle and the back electrode to cut out the ink to cause ink drop to be ejected onto a recording medium disposed between the recording needle and the back electrode. In this method, smaller ink drops may be formed in comparison with other methods because the ink meniscus is formed at the tip of the recording needle.
In addition, the Japanese Published Unexamined Patent Application No. Sho 56-28867 discloses an electrostatic extraction method that in the condition that the electrostatic field is applied between a recording needle electrode and a back electrode, image signal is applied to the needle electrode while at the same time the needle electrode is vibrated. In such a manner, the ink is said to be stably granulated.
However, the electrostatic method has its drawbacks such that the device becomes large and expensive, as the diameter of formed drop may vary in response to the variation of the thickness of the dielectric of the recording medium due to the humidity, and as supersonic also is required to be generated. In addition, high voltage regulating source is needed.
Therefore, the present invention provides a recording element and a recording device which uses no nozzle, may also eject very fine liquid drops, generate less heat and has higher energy efficiency by low frequency low voltage driving.
In the present invention, a recording element is provided,
wherein the recording element having an elastic member with a part thereof in contact with liquid surface, by vibrating the elastic member, drops of the liquid being ejected from a portion of the elastic member projected from the liquid surface.
A recording device for performing printing by ejecting liquid drops from a recording element to a surface to be printed is provided,
wherein the recording element having an elastic member with a part thereof in contact with liquid surface, by vibrating the elastic member, drops of the liquid being ejected from a portion of the elastic member projected from the liquid surface.
In the recording element of the present invention such constructed as mentioned above, by vibrating an elastic member, in the portion of the elastic member projected from the liquid surface, a large vibration energy is generated which may displace the projected portion while the liquid may be fed to the projected portion. Then the liquid supplied to the projected portion will be ejected as liquid drops, by the large vibration energy at the projected portion which may displace it.